D.Only nonedge ports moving to the forwarding state generate a TC BPDU.

E.If either an edge port or a nonedge port moves to a block state, then a TC BPDU is generated.

Explanation:

The IEEE 802.1D Spanning Tree Protocol was designed to keep a switched or bridged networkloop free, with adjustments made to the network topology dynamically. A topology change typicallytakes 30 seconds, where a port moves from the Blocking state to the Forwarding state after twointervals of the Forward Delay timer. As technology has improved, 30 seconds has become anunbearable length of time to wait for a production network to failover or "heal" itself during aproblem.Topology Changes and RSTPRecall that when an 802.1D switch detects a port state change (either up or down), it signals theRoot Bridge by sending topology change notification (TCN) BPDUs. The Root Bridge must thensignal a topology change by sending out a TCN message that is relayed to all switches in the STPdomain. RSTP detects a topology change only when a nonedge port transitions to the Forwardingstate. This might seem odd because a link failure is not used as a trigger. RSTP uses all of itsrapid convergence mechanisms to prevent bridging loops from forming.Therefore, topology changes are detected only so that bridging tables can be updated andcorrected as hosts appear first on a failed port and then on a different functioning port.When a topology change is detected, a switch must propagate news of the change to otherswitches in the network so they can correct their bridging tables, too. This process is similar to theconvergence and synchronization mechanism-topology change (TC) messages propagate throughthe network in an everexpanding wave.